SPIROSTOMUM AMBIGUUM 425 



unfortunately entailed a rather high death-rate, hut it seemed 

 unavoidahle. Further, the supply of material was meagre 

 and the technical difficulties considerable. 



For an hour oi- two su])sequent to the attachment there 

 was no change in the meganuclei or micronuclei of either 

 conjugant. 



The first big change to take place was the breaking up of the 

 meganucleus into isolated segments by the snapping of its com- 

 missures (Pis. 22 and 23, figs. 7 and 13). The greater number of 

 the segments of the meganucleus migrated towards the anterior 

 end of the conjugant's body and came to lie in the area opposite 

 to the Ihie of attachment. A few of the posterior segments 

 invariably remained in the neighbourhood of the contractile 

 vacuole and never migrated forwards. The fact that the mega- 

 nucleus became fragmented during conjugation was noticed 

 and figured by Stein (28). He did not describe it as actually 

 fragmenting, the pair of conjugants upon which he worked 

 evidently having passed this stage when he first observed them. 



In some preparations made before fragmentation of the 

 meganucleus and in almost all the ones made afterwards, 

 vacuoles were observed in the substance of the meganucleus. 

 These vacuoles varied in size, sometimes attaining to half 

 the size of the lobes of the meganucleus. Sometimes only 

 one vacuole was present in each lobe, but in other cases three 

 or four were present. The vacuoles often projected, causing 

 the nuclear membrane to bulge outwards. In stained prepara- 

 tions these vacuoles were colourless, but lightly staining spheres 

 could be seen in the centre of some of them (PI. 22, fig. 5, vac. 

 and p.). I am unable to offer any explanation of the nature 

 of these spheres. They were present at all stages from the 

 fragmentation of the meganucleus to the early stage of the 

 exconjugant. They were evidently a product of the degenera- 

 tion taking place in the meganucleus, and might result from 

 the coagulation by the fixative of some fluid in the vacuole. 

 This suggestion unfortunately is not very plausible, since it 

 demands either that the spheres should be present in all the 

 vacuoles, which was not the case, or else that the contents 



